"Game of Thrones" Season 7 is currently in production and it will showcase Daenerys Targaryen (Emilia Clarke) finally invading Westeros. She will be riding Drogon as she did in Season 6, but there are two other dragon riders that have yet to be revealed in the series.
It is widely speculated that the second of the dragon will be Jon Snow (Kit Harrington). The character has played a prominent role in the war against the White Walkers and most recently was named King in the North.
This will make sense in the context of the show given how Jon Snow has been revealed to be the son of Lyanna Stark (Aisling Franciosi) and Rhaegar Targaryen. This would make him Daenerys' nephew and would even give him a stronger claim to the Iron Throne.
Several months back, it was revealed that Jon Snow would be allying with Daenerys after she lands on Dragonstone, the former fortress of Stannis Baratheon (Stephen Dillane), according to leaked photos from TMZ. This would further indicate that Jon Snow would indeed become the second dragon rider.
Without Aegon on the show, the main mystery revolves around who will be the third head of the dragon. According to iDigital Times, the most likely candidates are both Tyrion Lannister (Peter Dinklage) and Euron Greyjoy (Pilou Asbæk).
In "Game of Thrones" Season 6, Euron revealed his plan to propose to Daenerys and give her his entire fleet in order to aid her conquer Westeros. However, it was also indicated he only aimed to use Daenerys for his own personal gain.
There are multiple fan theories suggesting Tyrion has Targaryen blood. Even if he does not it is stated that a person's will might suffice to tame a dragon and Tyrion did state multiple times on the show that he always dreamed of riding a dragon.
"Game of Thrones" Season 7 will premiere by summer 2017. No full-length trailer has yet been released but HBO has launched a behind-the-scenes video feature, which can be viewed below: